Semiconductor device with strain relaxed layer

ABSTRACT

A semiconductor device includes an epitaxial substrate. The epitaxial substrate includes a substrate. A strain relaxed layer covers and contacts the substrate. A III-V compound stacked layer covers and contacts the strain relaxed layer. The III-V compound stacked layer is a multilayer epitaxial structure formed by aluminum nitride, aluminum gallium nitride or a combination of aluminum nitride and aluminum gallium nitride.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 16/708,448,filed on Dec. 10th, 2019. The content of the application is incorporatedherein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a semiconductor device, and moreparticularly to a semiconductor device utilizing a strain relaxed layerto prevent stress generated by lattice mismatch.

2. Description of the Prior Art

Due to their semiconductor characteristics, III-V semiconductorcompounds may be applied in many kinds of integrated circuit devices,such as high power field effect transistors, high frequency transistors,or high electron mobility transistors. In the high electron mobilitytransistor, two semiconductor materials with different band-gaps arecombined and a heterojunction is formed at the junction between thesemiconductor materials as a channel for carriers. In recent years,gallium nitride based materials have been applied in high power and highfrequency products because of their properties of wider band-gap andhigh saturation velocity.

A two-dimensional electron gas (2DEG) may be generated by thepiezoelectric property of the GaN-based materials, and the switchingvelocity may be enhanced because of the higher electron velocity and thehigher electron density of the 2DEG.

However, because a lattice size of the III-V semiconductor compound anda lattice size of a substrate are greatly different, unwanted stress isgenerated in the III-V semiconductor compound. As a result, theefficiency of an HEMT will be decreased by this stress.

SUMMARY OF THE INVENTION

In light of the above, a strain relaxed layer is disposed between asubstrate and a III-V compound stacked layer to prevent the stresscaused by different lattice sizes which influences the III-V compoundstacked layer.

According to a preferred embodiment of the present invention, asemiconductor device includes an epitaxial substrate. The epitaxialsubstrate includes a substrate, a strain-relaxed layer covering andcontacting the substrate and a III-V compound stacked layer covering andcontacting the strain-relaxed layer. The III-V compound stacked layerincludes aluminum nitride, aluminum gallium nitride or a combination ofaluminum nitride and aluminum gallium nitride, and the III-V compoundstacked layer is a multilayer epitaxial structure.

According to another preferred embodiment of the present invention, asemiconductor device includes an epitaxial substrate. The epitaxialsubstrate includes a substrate, a nucleation layer covering andcontacting the substrate, a transition layer covering and contacting thenucleation layer, a superlattice covering and contacting the transitionlayer, and a strain-relaxed layer disposed in the superlattice. Thenucleation layer includes aluminum nitride, the transition layerincludes aluminum gallium nitride, the superlattice is formed byperiodically stacking aluminum gallium nitride and aluminum nitride andthe superlattice includes silicon oxide, silicon nitride or siliconcarbide.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a semiconductor substrate according to a first preferredembodiment of the present invention.

FIG. 2 depicts a semiconductor substrate according to a second preferredembodiment of the present invention.

FIG. 3 depicts a semiconductor substrate according to a third preferredembodiment of the present invention.

FIG. 4 depicts a schematic diagram of a strain-relaxed layer of thepresent invention.

FIG. 5 depicts an HEMT formed by using a semiconductor substrate in thefirst preferred embodiment of the present invention.

FIG. 6 depicts another HEMT formed by using a semiconductor substrate inthe first preferred embodiment of the present invention.

FIG. 7 depicts an HEMT formed by using a semiconductor substrate in thesecond preferred embodiment of the present invention.

FIG. 8 depicts another HEMT formed by using a semiconductor substrate inthe second preferred embodiment of the present invention.

FIG. 9 depicts an HEMT formed by using a semiconductor substrate in thethird preferred embodiment of the present invention.

FIG. 10 depicts another HEMT formed by using a semiconductor substratein the third preferred embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 depicts a semiconductor substrate according to a first preferredembodiment of the present invention.

As shown in FIG. 1, a semiconductor substrate 10 a includes an epitaxialsubstrate 12 and a device layer 14. The device layer 14 contacts andcovers the epitaxial substrate 12. The epitaxial substrate 12 includes asubstrate 16, a strain-relaxed layer 18, and a III-V compound stackedlayer 20. The substrate 16 includes a silicon substrate, a sapphiresubstrate or a silicon on insulator (SOI) substrate. The strain-relaxedlayer 18 covers and contacts the substrate 16. The strain-relaxed layer18 includes silicon oxide, silicon nitride, or silicon carbide. TheIII-V compound stacked layer 20 covers and contacts the strain-relaxedlayer 18. The III-V compound stacked layer 20 includes aluminum nitride,aluminum gallium nitride, gallium nitride, a combination of aluminumnitride and aluminum gallium nitride, a combination of aluminum nitrideand gallium nitride or a combination of aluminum nitride, aluminumgallium nitride and gallium nitride. Furthermore, the III-V compoundstacked layer 20 is a multilayer epitaxial structure. Moreover, theIII-V compound stacked layer 20 can include only one type of materialincluding aluminum nitride, aluminum gallium nitride or gallium nitride.On the other hand, the III-V compound stacked layer 20 can be acombination of aluminum nitride, aluminum gallium nitride and galliumnitride. The strain-relaxed layer 18 preferably does not include stress.Moreover, a thickness of the strain-relaxed layer 18 is greater than 1nanometer.

In details, in the first preferred embodiment, the III-V compoundstacked layer 20 includes a nucleation layer 22, a transition layer 24and a superlattice 26. The nucleation layer 22 covers and contacts thestrain-relaxed layer 18. The nucleation layer 22 preferably includesaluminum nitride. The transition layer 24 covers and contacts thenucleation layer 22. The transition layer 24 preferably includesaluminum gallium nitride. The superlattice 26 covers and contacts thetransition layer 24. The superlattice 26 is formed by periodicallystacking aluminum gallium nitride and aluminum nitride. Moreover, achemical formula of the aluminum gallium nitride in the transition layer24 is Al_(x)Ga_(1-x)N, and 0.7≤X≤0.8. The transition layer 24 can be asingle layer or multiple layers. If the transition layer 24 is multiplelayers, X can be altered to form numerous aluminum gallium nitridelayers which have different ratios of aluminum to gallium. A chemicalformula of the aluminum gallium nitride in the superlattice 26 isAl_(y)Ga_(1-y) N, and 0.2≤Y≤0.3. The superlattice 26 is formed byperiodically stacking aluminum gallium nitride and aluminum nitride.Furthermore, Y can be altered to form numerous aluminum gallium nitridelayers which have different ratios of aluminum to gallium. For example,the superlattice 26 may be a repeated stacked structure formed byAlN/Al_(0.2)Ga_(0.8)N, or a repeated stacked structure formed byAlN/Al_(0.2)Ga_(0.8)N/Al_(0.3)Ga_(0.7)N.

The device layer 14 includes a gallium nitride layer 28 and an aluminumgallium nitride layer 30 disposed on the gallium nitride layer 28. Analuminum nitride layer 32 can be optionally disposed between thealuminum gallium nitride layer 30 and the gallium nitride layer 28.

FIG. 2 depicts a semiconductor substrate according to a second preferredembodiment of the present invention, wherein elements which aresubstantially the same as those in the first preferred embodiment aredenoted by the same reference numerals; an accompanying explanation istherefore omitted.

As shown in FIG. 2, a semiconductor substrate 10 b includes an epitaxiallayer 12 and a device layer 14. The difference between the firstpreferred embodiment and the second preferred embodiment is that in thesecond preferred embodiment the superlattice 26 is replaced by agradient layer 126. Other elements in the second preferred embodimenthave the same position as that of the first preferred embodiment. Thegradient layer 126 covers and contacts the transition layer 24. Achemical formula of the gradient layer 126 is Al_(z)Ga_(1-z)N, 0≤Z 1,wherein Z decreases from a bottom of the gradient layer 126 to a top ofthe gradient layer 126. In other words, a value of Z in the gradientlayer 126 farther from the transition layer 24 is lower than a value ofZ in the gradient layer 126 closer to the transition layer 24. Thegradient layer 126 is a multiple-layered structure. The Z value in eachlayer of the multiple-layered structure is different from each other.For example, the gradient layer 126 may beAlN/Al_(0.8)Ga_(0.2)N/Al_(0.2)Ga_(0.8)N stacked from bottom to top. Thatis, the layer of AlN contacts the transition layer 24, the layer ofAl_(0.8)Ga_(0.2)N contacts the gallium nitride layer 28. However, thegradient layer 126 is not limited to the structure ofAlN/Al_(0.8)Ga_(0.2)N/Al_(0.2)Ga_(0.8)N. The number of layers in themultiple-layered structure, and the Z value can be altered.

Moreover, besides only one gradient layer 126, there can be numerousgradient layers 126. For example, if the gradient layer 126 has astructure of AlN/Al_(0.8)Ga_(0.2)N/Al_(0.2)Ga_(0.8)N, and there are twogradient layers 126, these two gradient layers 126 become a structure ofAlN/Al_(0.8)Ga_(0.2)N/Al_(0.2)Ga_(0.8)N stacked on a structure ofAlN/Al_(0.8)Ga_(0.2)N/Al_(0.2)Ga_(0.8)N.

FIG. 3 depicts a semiconductor substrate according to a third preferredembodiment of the present invention, wherein elements which aresubstantially the same as those in the first preferred embodiment aredenoted by the same reference numerals; an accompanying explanation istherefore omitted.

The difference between the first preferred embodiment and the thirdpreferred embodiment is that in the third preferred embodiment thestrain-relaxed layer 18 of the semiconductor substrate 10 c is insertedinto the superlattice 26 and separates the superlattice 26 into twoparts. In details, the epitaxial layer 12 in the third preferredembodiment includes substrate 16, the nucleation layer 22, thetransition layer 24 and the superlattice 26. The superlattice 26 coversand contacts the transition layer 24. The transition layer 24 covers andcontacts the nucleation layer 22. The nucleation layer 22 covers andcontacts the substrate 16. The strain-relaxed layer 18 is insertedbetween the repeated stacked structure of the superlattice 26. Forexample, if the superlattice 26 is formed by periodically stackingAlN/Al_(0.2)Ga_(0.8)N, the strain-relaxed layer 18 can be disposedbetween AlN and Al_(0.2)Ga_(0.8)N formed by any repeated cycle.

FIG. 4 depicts a schematic diagram of a strain-relaxed layer of thepresent invention. The strain-relaxed layers 18 shown in FIG. 4 can beapplied to the first preferred embodiment, the second preferredembodiment and the third preferred embodiment. As shown in FIG. 4, thestrain-relaxed layers 18 can has several types. The strain-relaxed layer18 at the topmost of FIG. 4 includes one first strain-relaxed layer 118.The first strain-relaxed layer 118 is single-layered. The firststrain-relaxed layer 118 includes silicon oxide, silicon nitride orsilicon carbide. For example, the first strain-relaxed layer 118 can besilicon nitride. Moreover, the first strain-relaxed layer 118 can alsobe multiple-layered from by different materials. For instance, the firststrain-relaxed layer 118 can be silicon carbide/silicon nitride.

Furthermore, as shown in the bottom left of FIG. 4, the strain-relaxedlayer 18 includes numerous first strain-relaxed layers 118. For example,if the first strain-relaxed layer 118 is silicon carbide/siliconnitride, two first strain-relaxed layers 118 have a structure of siliconcarbide/silicon nitride/silicon carbide/silicon nitride.

As shown in the bottom right of FIG. 4, besides the first strain-relaxedlayer 118, the strain-relaxed layer 18 can further include a secondstrain-relaxed layer 218 disposed on the first strain-relaxed layer 118.The second strain-relaxed layer 218 can be silicon oxide, siliconnitride or silicon carbide. The second strain-relaxed layer 218 can besingle-layered or multiple-layered. For example, the secondstrain-relaxed layer 218 can be a single-layered structure formed ofsilicon oxide or a multiple-layered formed of silicon nitride/siliconoxide.

Based on different requirement, different types of strain-relaxed layers18 in FIG. 4 can be applied to the epitaxial substrate 12 in the firstpreferred embodiment, the second preferred embodiment and the thirdpreferred embodiment.

Because the lattice size of the substrate 16 and the lattice size of thegallium nitride layer 28 are greatly different from each other, when thegallium nitride layer 28 directly contacts the substrate 16, stressesare respectively generated in the substrate 16 and the gallium nitridelayer 28 due to lattice mismatch. Theses stresses influence the qualityand efficiency of the semiconductor devices formed on the device layer14. The nucleation layer 22, the transition layer 22 and thesuperlattice 26 or the gradient layer 126 can compensate some of thelattice mismatch between the substrate 16 and the gallium nitride layer28. However, it is not good enough to solve the mismatch problem only bythe nucleation layer 22, the transition layer 22 and the superlattice 26or the gradient layer 126.

Therefore, a strain-relaxed layer 18 is disposed between the substrate16 and the gallium nitride layer 28. Because the lattice structure ofthe strain-relaxed layer 18 can relaxed stress, even placing thestrain-relaxed layer 18 on a material with large lattice mismatchcomparing to the strain-relaxed layer 18, only small stress or even nostress will be formed in the strain-relaxed layer 18. Therefore, whenplacing the strain-relaxed layer 18 between the substrate 16 and thegallium nitride layer 28, the lattice mismatch between the substrate 16and the strain-relaxed layer 18 will only generate small stress or evenno stress in the strain-relaxed layer 18. In this way, the galliumnitride layer 28 on the strain-relaxed layer 18 will not be influencedby the lattice mismatch. Briefly speaking, the strain-relaxed layer 18can prevent or decrease a material below the strain-relaxed layer 18from generating stress to a material on the strain-relaxed layer 18. Onthe other hand, the strain-relaxed layer 18 can prevent or decrease amaterial on the strain-relaxed layer 18 from generating stress to amaterial below the strain-relaxed layer 18.

In addition, the strain-relaxed layer 18 specially uses silicon oxide,silicon nitride or silicon carbide not only to compensate the stress dueto lattice mismatch, but also to make the III-V compound stacked layer20 a attached on the strain-relaxed layer 18 better, because thenucleation layer 22 or the superlattice 26 has a good attachment abilityto the strain-relaxed layer 18. Moreover, the band gap of the siliconoxide, silicon nitride or silicon carbide is larger than that of galliumnitride, therefore, the strain-relaxed layer 18 can increase a breakdownvoltage of a semiconductor device formed afterwards.

FIG. 5 depicts an HEMT formed by using a semiconductor substrate in thefirst preferred embodiment of the present invention, wherein elementswhich are substantially the same as those in the first preferredembodiment are denoted by the same reference numerals; an accompanyingexplanation is therefore omitted.

As shown in FIG. 5, an HEMT 100 a includes a semiconductor substrate 10a, a P-type gallium nitride layer 34 is disposed on an aluminum galliumnitride layer 30. A source electrode 36 and a drain electrode 38 aredisposed on the aluminum gallium nitride layer 30. The source electrode36 and the drain electrode 38 are respectively disposed at two sides ofthe P-type gallium nitride layer 34. A gate electrode 40 is disposedbetween the source electrode 36 and the drain electrode 38 and on thealuminum gallium nitride layer 30. The gate electrode 40 covers theP-type gallium nitride layer 34. A protective layer 42 conformallycovers the gate electrode 40, the source electrode 36, the drainelectrode 38 and the aluminum gallium nitride layer 30. The protectivelayer 42 includes gallium nitride or aluminum nitride. The HEMT 100 a isa normally-off transistor.

FIG. 6 depicts another HEMT formed by using a semiconductor substrate inthe first preferred embodiment of the present invention, whereinelements which are substantially the same as those in the firstpreferred embodiment are denoted by the same reference numerals; anaccompanying explanation is therefore omitted.

As shown in FIG. 6, an HEMT 100 b includes a semiconductor substrate 10b. A source electrode 36 and a drain electrode 38 are embedded in thealuminum gallium nitride layer 30. A gate electrode 40 is disposedbetween the source electrode 36 and the drain electrode 38 and embeddedin the aluminum gallium nitride layer 30. A protective layer 42conformally covers the source electrode 36, the drain electrode 38 andthe aluminum gallium nitride layer 30. The protective layer 42 isbetween the gate electrode 40 and the aluminum gallium nitride layer 30.The HEMT 100 b is a normally-off transistor.

FIG. 7 depicts an HEMT formed by using a semiconductor substrate in thesecond preferred embodiment of the present invention, wherein elementswhich are substantially the same as those in the second preferredembodiment are denoted by the same reference numerals; an accompanyingexplanation is therefore omitted.

The difference between the HEMT 200 a in FIG. 7 and the HEMT 100 a inFIG. 5 is that the superlattice 26 in FIG. 5 is replaced by a gradientlayer 126 in the semiconductor substrate 10 b in FIG. 7. Other elements,such as the P-type gallium nitride layer 34, the protective layer 42,the source electrode 36, the drain electrode 38 or the gate electrode40, have the same position as those in the HEMT 100 a in FIG. 5.Similarly, the HEMT 200 a is a normally-off transistor.

FIG. 8 depicts another HEMT formed by using a semiconductor substrate inthe second preferred embodiment of the present invention, whereinelements which are substantially the same as those in the secondpreferred embodiment are denoted by the same reference numerals; anaccompanying explanation is therefore omitted.

The difference between the HEMT 200 b in FIG. 8 and the HEMT 100 b inFIG. 6 is that the superlattice 26 in FIG. 6 is replaced by a gradientlayer 126 in the semiconductor substrate 10 b in FIG. 8. Other elements,such as the protective layer 42, the source electrode 36, the drainelectrode 38 or the gate electrode 40, have the same position as thosein the HEMT 100 b in FIG. 6. Similarly, the HEMT 200 b is a normally-offtransistor.

FIG. 9 depicts an HEMT formed by using a semiconductor substrate in thethird preferred embodiment of the present invention, wherein elementswhich are substantially the same as those in the third preferredembodiment are denoted by the same reference numerals; an accompanyingexplanation is therefore omitted.

The difference between the HEMT 300 a in FIG. 9 and the HEMT 100 a inFIG. 5 is that the strain-relaxed layer 18 is inserted into thesuperlattices 26 in FIG. 9. Other elements, such as the P-type galliumnitride layer 34, the protective layer 42, the source electrode 36, thedrain electrode 38 or the gate electrode 40, have the same position asthose in the HEMT 100 a in FIG. 5. Similarly, the HEMT 300 a is anormally-off transistor.

FIG. 10 depicts another HEMT formed by using a semiconductor substratein the third preferred embodiment of the present invention, whereinelements which are substantially the same as those in the thirdpreferred embodiment are denoted by the same reference numerals; anaccompanying explanation is therefore omitted.

The difference between the HEMT 300 b in FIG. 100 and the HEMT 100 b inFIG. 6 is that that the strain-relaxed layer 18 is inserted into thesuperlattices 26 in FIG. 9. Other elements, such as the protective layer42, the source electrode 36, the drain electrode 38 or the gateelectrode 40, have the same position as those in the HEMT 100 b in FIG.6. Similarly, the HEMT 300 b is a normally-off transistor.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A semiconductor device, comprising: an epitaxialsubstrate comprising: a substrate; a nucleation layer covering andcontacting the substrate, wherein the nucleation layer comprisesaluminum nitride; a transition layer covering and contacting thenucleation layer, wherein the transition layer comprises aluminumgallium nitride; a superlattice covering and contacting the transitionlayer, wherein the superlattice is formed by periodically stackingaluminum gallium nitride and aluminum nitride; a strain-relaxed layerinserted into the superlattice, wherein the superlattice comprisessilicon oxide, silicon nitride or silicon carbide; and a gate electrodedisposed on a protective layer and embedded within an aluminum galliumnitride layer, wherein the protective layer is disposed on the epitaxialsubstrate.
 2. The semiconductor device of claim 1, wherein a chemicalformula of the aluminum gallium nitride in the transition layer isAl_(x)Ga_(1-x)N, and 0.7≤X≤0.8.
 3. The semiconductor device of claim 1,wherein a chemical formula of the aluminum gallium nitride in thesuperlattice is Al_(y)Ga_(1-y)N, and 0.2≤Y≤0.3.
 4. The semiconductordevice of claim 1, further comprising: a device layer contacting andcovering the epitaxial substrate, wherein the device layer comprises: agallium nitride layer; and the aluminum gallium nitride layer disposedon the gallium nitride layer.
 5. The semiconductor device of claim 4,further comprising: an HEMT disposed on the epitaxial substrate, whereinthe HEMT comprises: the epitaxial substrate; the device layer; a sourceelectrode and a drain electrode disposed on the aluminum gallium nitridelayer; and the gate electrode disposed between the source electrode andthe drain electrode.
 6. The semiconductor device of claim 1, wherein athickness of the strain-relaxed layer is greater than 1 nanometer.